Multicolor organic light emitting devices

ABSTRACT

A multicolor organic light emitting device employs vertically stacked layers of double heterostructure devices which are fabricated from organic compounds. The vertical stacked structure is formed on a glass base having a transparent coating of ITO or similar metal to provide a substrate. Deposited on the substrate is the vertical stacked arrangement of three double heterostructure devices, each fabricated from a suitable organic material. Stacking is implemented such that the double heterostructure with the longest wavelength is on the top of the stack. This constitutes the device emitting red light on the top with the device having the shortest wavelength, namely, the device emitting blue light, on the bottom of the stack. Located between the red and blue device structures is the green device structure. The devices are configured as stacked to provide a staircase profile whereby each device is separated from the other by a thin transparent conductive contact layer to enable light emanating from each of the devices to pass through the semitransparent contacts and through the lower device structures while further enabling each of the devices to receive a selective bias.

FIELD OF THE INVENTION

This invention relates to multicolor organic light emitting devices andmore particularly to such devices for use in flat panel electronicdisplays.

BACKGROUND OF THE INVENTION

The electronic display is an indispensable way in modern society todeliver information and is utilized in television sets, computerterminals and in a host of other applications. No other medium offersits speed, versatility and interactivity. Known display technologiesinclude plasma displays, light emitting diodes (LEDs), thin filmelectroluminescent displays, and so forth.

The primary non-emissive technology makes use of the electro opticproperties of a class of organic molecules known as liquid crystals(LCs) or liquid crystal displays (LCDs). LCDs operate fairly reliablybut have relatively low contrast and resolution, and require high powerbacklighting. Active matrix displays employ an array of transistors,each capable of activating a single LC pixel. There is no doubt that thetechnology concerning flat panel displays is of a significant concernand progress is continuously being made. See an article entitled "FlatPanel Displays", Scientific American, March 1993, pgs. 90-97 by S. W.Depp and W. E. Howard. In that article, it is indicated that by 1995flat panel displays alone are expected to form a market of between 4 and5 billion dollars. Desirable factors for any display technology is theability to provide a high resolution full color display at good lightlevel and at competitive pricing.

Color displays operate with the three primary colors red (R), green (G)and blue (B). There has been considerable progress in demonstrating red,green and blue light emitting devices (LEDs) using organic thin filmmaterials. These thin film materials are deposited under high vacuumconditions. Such techniques have been developed in numerous placesthroughout the world and this technology is being worked on in manyresearch facilities.

Presently, the most favored high efficiency organic emissive structureis referred to as the double heterostructure LED which is shown in FIG.1A and designated as prior art. This structure is very similar toconventional, inorganic LED's using materials as GaAs or InP.

In the device shown in FIG. 1A, a support layer of glass 10 is coated bya thin layer of Indium Tin Oxide (ITO) 11, where layers 10 and 11 formthe substrate. Next, a thin (100-500 Å) organic, predominantly holetransporting layer (HTL) 12 is deposited on the ITO layer 11. Depositedon the surface of HTL layer 12 is a thin (typically, 50 Å-100Å) emissionlayer (EL) 13. If the layers are too thin there may be lack ofcontinuity in the film, and thicker films tend to have a high internalresistance requiring higher power operation. Emissive layer (EL) 13provides the recombination site for electrons injected from a 100 Å-500Å thick electron transporting layer 14 (ETL) with holes from the HTLlayer 12. The ETL material is characterized by its considerably higherelectron than hole mobility. Examples of prior art ETL, EL and HTLmaterials are disclosed in U.S. Pat. No. 5,294,870 entitled "OrganicElectroluminescent MultiColor Image Display Device", issued on Mar. 15,1994 to Tang et al.

Often, the EL layer 13 is doped with a highly fluorescent dye to tunecolor and increase the electroluminescent efficiency of the LED. Thedevice as shown in FIG. 1A is completed by depositing metal contacts 15,16 and top electrode 17. Contacts 15 and 16 are typically fabricatedfrom indium or Ti/Pt/Au. Electrode 17 is often a dual layer structureconsisting of an alloy such as Mg/Ag 17' directly contacting the organicETL layer 14, and a thick, high work function metal layer 17" such asgold (Au) or silver (Ag) on the Mg/Ag. The thick metal 17" is opaque.When proper bias voltage is applied between top electrode 17 andcontacts 15 and 16, light emission occurs through the glass substrate10. An LED device of FIG. 1A typically has luminescent external quantumefficiencies of from 0.05 percent to 4 percent depending on the color ofemission and its structure.

Another known organic emissive structure referred as a singleheterostructure is shown in FIG. 1B and designated as prior art. Thedifference in this structure relative to that of FIG. 1A, is that the ELlayer 13 serves also as an ETL layer, eliminating the ETL layer 14 ofFIG. 1A. However, the device of FIG. 1B, for efficient operation, mustincorporate an EL layer 13 having good electron transport capability,otherwise a separate ETL layer 14 must be included as shown for thedevice of FIG. 1A.

Presently, the highest efficiencies have been observed in green LED's.Furthermore, drive voltages of 3 to 10 volts have been achieved. Theseearly and very promising demonstrations have used amorphous or highlypolycrystalline organic layers. These structures undoubtedly limit thecharge carrier mobility across the film which, in turn, limits currentand increases drive voltage. Migration and growth of crystallitesarising from the polycrystalline state is a pronounced failure mode ofsuch devices. Electrode contact degradation is also a pronounced failuremechanism.

Yet another known LED device is shown in FIG. 1C, illustrating a typicalcross sectional view of a single layer (polymer) LED. As shown, thedevice includes a glass support layer 1, coated by a thin ITO layer 3,for forming the base substrate. A thin organic layer 5 of spin-coatedpolymer, for example, is formed over ITO layer 3, and provides all ofthe functions of the HTL, ETL, and EL layers of the previously describeddevices. A metal electrode layer 6 is formed over organic layer 5. Themetal is typically Mg, Ca, or other conventionally used metals.

An example of a multicolor electroluminescent image display deviceemploying organic compounds for light emitting pixels is disclosed inTang et al., U.S. Pat. No. 5,294,870. This patent discloses a pluralityof light emitting pixels which contain an organic medium for emittingblue light in blue-emitting subpixel regions. Fluorescent media arelaterally spaced from the blue-emitting subpixel region. The fluorescentmedia absorb light emitted by the organic medium and emit red and greenlight in different subpixel regions. The use of materials doped withfluorescent dyes to emit green or red on absorption of blue light fromthe blue subpixel region is less efficient than direct formation viagreen or red LED's. The reason is that the efficiency will be theproduct of (quantum efficiency for EL)*(quantum efficiency forfluorescence)*(1-transmittance). Thus a drawback of this display is thatdifferent laterally spaced subpixel regions are required for each coloremitted.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multicolor organiclight emitting device employing several types of organicelectroluminescent media, each for emitting a distinct color.

It is a further object of this invention to provide such a device in ahigh definition multicolor display in which the organic media arearranged in a stacked configuration such that any color can be emittedfrom a common region of the display.

It is another object of the present invention to provide a three colororganic light emitting device which is extremely reliable and relativelyinexpensive to produce.

It is a further object to provide such a device which is implemented bythe growth of organic materials similar to those materials used inelectroluminescent diodes, to obtain an organic LED which is highlyreliable, compact, efficient and requires low drive voltages forutilization in RGB displays.

In one embodiment of the invention, a multicolor light emitting device(LED) structure comprises at least a first and a second organic LEDstacked one upon the other, and preferably three, to form a layeredstructure, with each LED separated one from the other by a transparentconductive layer to enable each device to receive a separate biaspotential to emit light through the stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross sectional view of a typical organic doubleheterostructure light emitting device (LED) according to the prior art.

FIG. 1B is a cross sectional view of a typical organic singleheterostructure light emitting device (LED) according to the prior art.

FIG. 1C is a cross sectional view of a known single layer polymer LEDstructure according to the prior art.

FIGS. 2A, 2B, and 2C are cross sectional views of an integrated threecolor pixel utilizing crystalline organic light emitting devices(LED's), respectively, according to embodiments of this invention,respectively.

FIGS. 3-11 show a variety of organic compounds which may be used tocomprise the active emission layers for generating the various colors.

FIGS. 12(A-E) illustrate a shadow masking process for the fabrication ofthe multicolor LED according to the invention.

FIGS. 13(A-F) illustrate a dry etching process for the fabrication ofthe multicolor LED according to the invention.

FIG. 14A shows a multicolor LED of one embodiment of this inventionconfigured for facilitating packaging thereof.

FIG. 14B shows a cross sectional view of a hermetic package for anotherembodiment of the invention.

FIG. 14C is cross sectional view taken along 14C--14C of FIG. 14B.

FIG. 15 is a block diagram showing an RGB display utilizing LED devicesaccording to this invention together with display drive circuitry.

FIG. 16 shows an LED device of another embodiment of the inventionextending the number of stacked LED's to N, where N is an integer number1, 2, 3, . . . N.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A has been described and is a prior art double heterostructureorganic light emitting device. The basic construction of the device ofFIG. 1A is used in this invention as will be described.

Referring to FIG. 2A, there is shown a schematic cross section of ahighly compact, integrated RGB pixel structure which is implemented bygrown or vacuum deposited organic layers, in one embodiment of theinvention. Based on the ability to grow organic materials on a largevariety of materials (including metals and ITO), one can construct astack of LED double heterostructures (DH) designated as 20, 21 and 22,in one embodiment of the invention. For illustrative purposes, LED 20 isconsidered in a bottom portion of the stack, LED 21 in a middle portionof the stack, and LED 22 in a top portion of the stack, in the exampleof FIG. 2A. Also, the stack is shown to be vertically oriented in FIG.2A, but the LED can be otherwise oriented. In other embodiments, a stackof single heterostructure (SH) LED's (see FIG. 1B), or a stack ofpolymer-based LED devices (see FIG. 1C), are viable alternatives to theDH LED's, with the SH devices being equally viable as DH devices forlight emitters. Also, SH and DH devices comprising a combination ofvacuum deposited and polymeric light-emitting materials are consideredto be within the spirit and scope of this invention.

Each device structure as device 20, consists of an HTL layer 20Hvacuum-deposited or grown on or otherwise deposited onto the surface ofan ITO layer 35. A top ETL layer 20T sandwiches an EL layer 20E betweenthe former and HTL layer 20H, for example, shown in the deviceconstruction of FIG. 2A. The ETL layer 20T and other ETL layers to bedescribed are composed of organic materials such asM(8-hydroxyquinolate)_(n) (M=metal ion; n=2-4). Examples of othersuitable organic ETL materials can be found in U.S. Pat. No. 5,294,870to Tang et al. Formed on top of ETL layer 20T is a thin,semi-transparent low work function (preferably, <4 eV) metal layer 26Mhaving a thickness typically less than 50 Å. Suitable candidates includeMg, Mg/Ag, and As. Deposited on the top of metal layer 26M is anothertransparent, thin conductive ITO layer 26I. (For convenience herein, thedouble layer structure of metallic layer 26M and ITO layer 26I isreferred to as ITO/metal layers 26.) Each of the double heterostructuredevices as 20, 21 and 22 have a bottom HTL layer formed on a transparentconductive layer of ITO 26I or 35. Next an EL layer is deposited andthen another layer of ETL. Each of the HTL, ETL, ITO, metal and organicEL layers are transparent because of their composition and minimalthickness. Each HTL layer may be 50 -1000 Å thick; each EL layer may be50 -200 Å thick; each ETL layer may be 50 -1000 Å thick; each metallayer 26M may be 50 -100 Å thick; and each ITO layer 26I and 35 may be1000 -4000 Å thick. For optimum performance, each of the layers shouldpreferably be kept towards the lower ends of the above ranges. Thus,each LED 20, 21 and 22 (excluding ITO/metal layers) are preferably closeto 200 Å thick.

If SH LED devices are used for providing LED's 20, 21, 22, rather thanDH LED devices, the ETL and EL layers are provided by a single layer,such as layer 13, as previously described for the SH of FIG. 1B. Thislayer 13 is typically Alquinolate. This is shown in FIG. 2B, where theEL layers 20E, 21E, and 22E, respectively, provide both the EL and ETLlayer functions. However, an advantage of the DH LED stack of FIG. 2A,relative to a SH LED stack of FIG. 2B, is that the DH LED stack permitsthinner overall construction with high efficiency.

In FIGS. 2A and 2B, even though the centers of each of the LED's areoffset from one another, the total beam of light from each device issubstantially coincident between LED's 20, 21 and 22. While the beams oflight are coincident in the concentric configuration, the emitting ornon-emitting device closer to the glass substrate will be transparent tothe emitting device or devices further away from the glass substrate.However, the diodes 20, 21 and 22 need not be offset from one anotherand may alternatively be stacked concentrically upon each other,whereupon the beam of light from each device is wholly coincident withthe others. A concentric configuration is shown in FIG. 12E which willbe described below in regard to device fabrication processes. Note thatthere is no difference in function between the offset and concentricconfigurations. Each device emits light through glass substrate 37 in asubstantially omnidirectional pattern. The voltages across the threeLED's in the stack 29 are controlled to provide a desired resultantemission color and brightness for the particular pixel at any instant oftime. Thus, each LED as 22, 21 and 20 can be energized simultaneouslywith beams as R, G and B, respectively, for example, directed throughand visible via the transparent layers, as shown schematically in FIGS.2A and 2B. Each DH structure 20, 21 and 22 is capable upon applicationof a suitable bias voltage of emitting a different color light. Thedouble heterostructure LED 20 emits blue light. The doubleheterostructure LED 21 emits green light while the doubleheterostructure (DH) LED 22 emits red light. Different combinations orindividual ones of LED's 20, 21 and 22 can be activated to selectivelyobtain a desired color of light for the respective pixel partlydependent upon the magnitude of current in each of the LED's 20, 21 and22.

In the example of FIGS. 2A and 2B, LED's 20, 21 and 22 are forwardbiased by batteries 32, 31 and 30, respectively. Current flows from thepositive terminal of each battery 32, 31 and 30, into the anode terminal40, 41, 42, respectively, of its associated LED 20, 21 and 22,respectively, through the layers of each respective device, and fromterminals 41, 42 and 43, serving as cathode terminals to negativeterminals of each battery 32, 31, and 30, respectively. As a result,light is emitted from each of the LED's 20, 21 and 22. The LED devices20, 21 and 22 are made selectively energizable by including means (notshown) for selectively switching batteries 32, 31 and 30, respectively,into and out of connection to their respective LED.

In the embodiments of the invention, relative to FIGS. 2A and 2B, thetop ITO contact 26I for LED 22 is transparent, making the three colordevice shown useful for headup display applications. However, in anotherembodiment of the invention, the top contact 26I is formed from a thickmetal, such as either Mg/Ag, In, Ag, or Au, for reflecting light emittedupward back through substrate 13, for substantially increasing theefficiency of the device. Also, overall device efficiency can beincreased by forming a multilayer dielectric thin film coating betweenglass substrate 37 and the ITO layer 35, to provide an anti-reflectingsurface. Three sets of anti-reflecting layers are required, one to forman anti-reflection coating at each wavelength emitted from the variouslayers.

In another embodiment, the device of FIG. 2A is constructed in anopposite or inverted manner, for providing light emission out of the topof stack rather than the bottom as the former. An example of an invertedstructure, with reference to FIG. 2C, is to replace ITO layer 35 with athick, reflective metal layer 38. Blue LED 20 is then provided byinterchanging HTL layer 20H and ETL layer 20T, with EL layer 20Eremaining sandwiched between the latter two layers. Furthermore, themetal contact layer 26M is now deposited on top of ITO layer 26I. Thegreen LED 21 and red LED 22 portions of the stack are each constructedwith inverted layers (the HTL and ETL layers of each are interchanged,followed by inverting the metal and ITO layers) as described for theinverted blue LED 20. Note that in the inverted structure, the bluedevice 20 must be on top and the red device 22 on the bottom. Also, thepolarities of batteries 30, 31, and 32 are reversed. As a result, thecurrent flow through devices 20, 21 and 22, respectively, is in the samedirection relative to the embodiment of FIG. 2A, when forward biased foremitting light.

The device in the cross sectional view has a step-like or staircaseprofile, in this example. The transparent contact areas (ITO) 26I permitseparate biasing of each pixel element in the stack and furthermore thematerial can be used as an etch stop during the processing steps. Theseparate biasing of each DH LED structure 20, 21 and 22 allows forwavelength tuning of the pixel output to any of various desired colorsof the visible spectrum as defined in the CIE (Commission Internationalede l'Eclairage/International Commission of Illumination) chromaticitystandard. The blue emitting LED 20 is placed at the bottom of the stackand it is the largest of the three devices. Blue is on the bottombecause it is transparent to red and green light. Finally, the materials"partitioning" using the transparent ITO/metal layers 26 facilitatesmanufacture of this device as will be described. It is the very uniqueaspects of the vacuum growth and fabrication processes associated withorganic compounds which makes the pixel LED devices shown in FIGS. 2A,2B, and 2C possible. The vertical layering shown in FIGS. 2A, 2B, and 2Callows for the fabrication of three color pixels with the smallestpossible area, hence, making these ideal for high definition displays.

As seen in FIGS. 2A, 2B, and 2C, each device DH structure 20, 21 and 22can emit light designated by arrows B, G and R, respectively, eithersimultaneously or separately. Note that the emitted light is fromsubstantially the entire transverse portion of each LED 20, 21 and 22,whereby the R, G, and B arrows are not representative of the width ofthe actual emitted light, respectively. In this way, the addition orsubtraction of colors as R, G and B is integrated by the eye causingdifferent colors and hues to be perceived. This is well known in thefield of color vision and display colorimetry. In the offsetconfiguration shown, the red, green and blue beams of light aresubstantially coincident. If the devices are made small enough, that isabout 50 microns or less, any one of a variety of colors can be producedfrom the stack. However, it will appear as one color originating from asingle pixel.

The organic materials used in the DH structures are grown one on top ofthe other or are vertically stacked with the longest wavelength device22 indicative of red light on the top and the shortest wavelengthelement 20 indicative of blue light on the bottom. In this manner, oneminimizes light absorption in the pixel or in the devices. Each of theDH LED devices are separated by ITO/metal layers 26 (specifically,semitransparent metal layers 26M, and indium tin oxide layers 26I). TheITO layers 26I can further be treated by metal deposition to providedistinct contact areas on the exposed ITO surfaces, such as contacts 40,41, 42 and 43. These contacts 40, 41, 42 and 43 are fabricated fromindium, platinum, gold, silver or alloys such as Ti/Pt/Au, Cr/Au, orMg/Ag, for example. Techniques for deposition of contacts usingconventional metal deposition or vapor deposition are well known. Thecontacts, such as 40, 41, 42 and 43, enable separate biasing of each LEDin the stack. The significant chemical differences between the organicLED materials and the transparent electrodes 26I permits the electrodesto act as etch stop layers. This allows for the selective etching andexposure of each pixel element during device processing.

Each LED 20, 21, 22 has its own source of bias potential, in thisexample shown schematically as batteries 32, 31, and 30, respectively,which enables each LED to emit light. It is understood that suitablesignals can be employed in lieu of the batteries 30, 31, 32,respectively. As is known, the LED requires a minimum threshold voltageto emit light (each DH LED) and hence this activating voltage is shownschematically by the battery symbol.

The EL layers 20E, 21E, 22E may be fabricated from organic compoundsselected according to their ability to produce all primary colors andintermediates thereof. The organic compounds are generally selected fromtrivalent metal quinolate complexes, trivalent metal bridged quinolatecomplexes, Schiff base divalent metal complexes, tin (iv) metalcomplexes, metal acetylacetonate complexes, metal bidentate ligandcomplexes, bisphosphonates, divalent metal maleonitriledithiolatecomplexes, molecular charge transfer complexes, aromatic andheterocyclic polymers and rare earth mixed chelates, as describedhereinafter.

The trivalent metal quinolate complexes are represented by thestructural formula shown in FIG. 3, wherein M is a trivalent metal ionselected from Groups 3-13 of the Periodic Table and the Lanthanides.Al⁺³, Ga⁺³ and In⁺³ are the preferred trivalent metal ions.

R of FIG. 3 includes hydrogen, substituted and unsubstituted alkyl, aryland heterocyclic groups. The alkyl group may be straight or branchedchain and preferably has from 1 to 8 carbon atoms. Examples of suitablealkyl groups are methyl and ethyl. The preferred aryl group is phenyland examples of the heterocyclic group for R include pyridyl, imidazole,furan and thiophene.

The alkyl, aryl and heterocyclic groups of R may be substituted with atleast one substituent selected from aryl, halogen, cyano and alkoxy,preferably having from 1 to 8 carbon atoms. The preferred halogen ischloro.

The group L of FIG. 3 represents a ligand including picolylmethylketone,substituted and unsubstituted salicylaldehyde (e.g. salicylaldehydesubstituted with barbituric acid), a group of the formula R(O)CO--wherein R is as defined above, halogen, a group of the formula R0--wherein R is as defined above, and quinolates (e.g. 8-hydroxyquinoline)and derivatives thereof (e.g. barbituric acid substituted quinolates).Preferred complexes covered by the formula shown in FIG. 3 are thosewhere M is Ga⁺³ and L is chloro. Such compounds generate a blueemission. When M is Ga⁺³ and L is methyl carboxylate, complexes emittingin the blue to blue/green region are produced. A yellow or red emissionis expected by using either a barbituric acid substitutedsalicylaldehyde or a barbituric acid substituted 8-hydroxyquinoline forthe L group. Green emissions may be produced by using a quinolate forthe L group.

The trivalent metal bridged quinolate complexes which may be employed inthe present invention are shown in FIGS. 4A and 4B. These complexesgenerate green emissions add exhibit superior environmental stabilitycompared to trisquinolates (complexes of FIG. 3 where L is a quinolate)used in prior art devices. The trivalent metal ion M used in thesecomplexes is as defined above with Al⁺³, Ga⁺³, or In⁺³ being preferred.The group Z shown in FIG. 4A has the formula SiR wherein R is as definedabove. Z may also be a group of the formula P═O which forms a phosphate.

The Schiff base divalent metal complexes include those shown in FIGS. 5Aand 5B wherein M¹ is a divalent metal chosen from Groups 2-12 of thePeriodic Table, preferably Zn (See, Y. Hanada, et al., "BlueElectroluminescence in Thin Films of Axomethin--Zinc Complexes",Japanese Journal of Applied Physics Vol. 32, pp. L511-L513 (1993). Thegroup R¹ is selected from the structural formulas shown in FIGS. 5A and5B. The R^(l) group is preferably coordinated to the metal of thecomplex through the amine or nitrogen of the pyridyl group. X isselected from hydrogen, alkyl, alkoxy, each having from 1 to 8 carbonatoms, aryl, a heterocyclic group, phosphino, halide and amine. Thepreferred aryl group is phenyl and the preferred heterocyclic group isselected from pyridyl, imidazole, furan and thiophene. The X groupsaffect the solubility of the Schiff base divalent metal complexes inorganic solvents. The particular Schiff base divalent metal complexshown in FIG. 5B emits at a wavelength of 520 nm.

The tin (iv) metal complexes employed in the present invention in the ELlayers generate green emissions. Included among these complexes arethose having the formula SnL¹ ₂ L² ₂ where L¹ is selected fromsalicylaldehydes, salicyclic acid or quinolates (e.g.8-hydroxyquinoline). L² includes all groups as previously defined for Rexcept hydrogen. For example, tin (iv) metal complexes where L¹ is aquinolate and L² is phenyl have an emission wavelength (λ_(em)) of 504nm, the wavelength resulting from measurements of photoluminescence inthe solid state.

The tin (iv) metal complexes also include those having the structuralformula of FIG. 6 wherein Y is sulfur or NR² where R² is selected fromhydrogen and substituted or unsubstituted, alkyl and aryl. The alkylgroup may be straight or branched chain and preferably has from 1 to 8carbon atoms. The preferred aryl group is phenyl. The substituents forthe alkyl and aryl groups include alkyl and alkoxy having from 1 to 8carbon atoms, cyano and halogen. L³ may be selected from alkyl, aryl,halide, quinolates (e.g. 8-hydroxyquinoline), salicylaldehydes,salicylic acid, and maleonitriledithiolate ("mnt"). When A is S and Y isCN and L³ is "mnt" an emission between red and orange is expected.

The M(acetylacetonate)₃ complexes shown in FIG. 7 generate a blueemission. The metal ion M is selected from trivalent metals of Groups3-13 of the Periodic Table and the Lanthanides. The preferred metal ionsare A1⁺³, Ga⁺³ and In⁺³. The group R in FIG. 7 is the same as definedfor R in FIG. 3. For example, when R is methyl, and M is selected fromAl⁺³, Ga⁺³ and In⁺³, respectively, the wavelengths resulting from themeasurements of photoluminescence in the solid state is 415 nm, 445 nmand 457 nm, respectively (See J. Kido et al., "OrganicElectroluminescent Devices using Lanthanide Complexes", Journal ofAlloys and Compounds, Vol. 92, pp. 30-33 (1993).

The metal bidentate complexes employed in the present inventiongenerally produce blue emissions.

Such complexes have the formula MDL₄ ₂ wherein M is selected fromtrivalent metals of Groups 3-13 of the Periodic Table and theLanthanides. The preferred metal ions are Al⁺³, Ga⁺³, In⁺³ and Sc⁺³. Dis a bidentate ligand examples of which are shown in FIG. 8A. Morespecifically, the bidentate ligand D includes 2-picolylketones,2-quinaldylketones and 2-(o-phenoxy) pyridine ketones where the R groupsin FIG. 8A are as defined above.

The preferred groups for L⁴ include acetylacetonate; compounds of theformula OR³ R wherein R³ is selected from Si, C and R is selected fromthe same groups as described above; 3, 5-di(t-bu) phenol; 2, 6-di(t-bu)phenol; 2, 6-di(t-bu) cresol; and H₂ Bpz₂, the latter compounds beingshown in FIGS. 8B-8E, respectively.

By way of example, the wavelength (λ_(em)) resulting from measurement ofphotoluminescence in the solid state of aluminum (picolymethylketone)bis 2, 6-di(t-bu) phenoxide! is 420 nm. The cresol derivative of theabove compound also measured 420 nm. Aluminum (picolylmethylketone) bis(OSiPh₃) and scandium (4-methoxy-picolylmethylketone) bis(acetylacetonate) each measured 433 nm, while aluminum2-(O-phenoxy)pyridine! bis 2, 6-di(t-bu) phenoxide! measured 450 nm.

Bisphosphonate compounds are another class of compounds which may beused in accordance with the present invention for the EL layers. Thebisphosphonates are represented by the general formula:

    M.sup.2.sub.x (O.sub.3 P-organic-PO.sub.3).sub.y

M^(r) is a metal ion. It is a tetravalent metal ion (e.g. Zr⁺⁴, Ti⁺⁴ andHf⁺⁴ when x and y both equal 1. When x is 3 and y is 2, the metal ion M²is in the divalent state and includes, for example, Zn⁺², Cu⁺² and Cd⁺³.The term "organic" as used in the above formula means any aromatic orheterocyclic fluorescent compound that can be bifunctionalized withphosphonate groups.

The preferred bisphosphonate compounds include phenylene vinylenebisphonsphonates as for example those shown in FIGS. 9A and 9B.Specifically, FIG. 9A shows β-styrenyl stilbene bisphosphonates and FIG.9B shows 4, 4'-biphenyl di(vinylphosphonates) where R is as describedpreviously and R⁴ is selected from substituted and unsubstituted alkylgroups, preferably having 1-8 carbon atoms, and aryl. The preferredalkyl groups are methyl and ethyl. The preferred aryl group is phenyl.The preferred substituents for the alkyl and aryl groups include atleast one substituent selected from aryl, halogen, cyano, alkoxy,preferably having from 1 to 8 carbon atoms.

The divalent metal maleonitriledithiolate ("mnt") complexes have thestructural formula shown in FIG. 10. The divalent metal ion M³ includesall metal ions having a +2 charge, preferably transition metal ions suchas Pt⁺², Zn⁺² and Pd⁺². Y¹ is selected from cyano and substituted orunsubstituted phenyl. The preferred substituents for phenyl are selectedfrom alkyl, cyano, chloro and 1, 2, 2-tricyanovinyl.

L⁵ represents a group having no charge. Preferred groups for L⁵ includeP(OR)₃ and P(R)₃ where R is as described above or L⁵ may be a chelatingligand such as, for example, 2, 2'-dipyridyl; phenanthroline; 1,5-cyclooctadiene; or bis(diphenylphosphino)methane.

Illustrative examples of the emission wavelengths of variouscombinations of these compounds are shown in Table 1, as derived from C.E. Johnson et al., "Luminescent Iridium(I), Rhodium(I), and Platinum(II)Dithiolate Complexes", Journal of the American Chemical Society, Vol.105, pg. 1795 (1983).

                  TABLE 1                                                         ______________________________________                                        Complex                  Wavelength*                                          ______________________________________                                         Platinum(1,5-cyclooctadiene) (mnt)!                                                                   560 nm                                                Platinum(P(OEt).sub.3).sub.2 (mnt)!                                                                   566 nm                                                Platinum(P(OPh).sub.3).sub.2 (mnt)!                                                                   605 nm                                                Platinum(bis(diphenylphosphino)methane) (mnt)!                                                        610 nm                                                Platinum(PPh.sub.3).sub.2 (mnt)!                                                                      652 nm                                               ______________________________________                                         *wavelength resulting from measurement of photoluminescence in the solid      state.                                                                   

Molecular charge transfer complexes employed in the present inventionfor the EL layers are those including an electron acceptor structurecomplexed with an electron donor structure. FIGS. 11A-11E show a varietyof suitable electron acceptors which may form a charge transfer complexwith one of the electron donor structures shown in FIGS. 11F-11J. Thegroup R as shown in FIGS. 11A and 11H is the same as described above.

Films of these charge transfer materials are prepared by eitherevaporating donor and acceptor molecules from separate cells onto thesubstrate, or by evaporating the pre-made charge transfer complexdirectly. The emission wavelengths may range from red to blue, dependingupon which acceptor is coupled with which donor.

Polymers of aromatic and heterocyclic compounds which are fluorescent inthe solid state may be employed in the present invention for the ELLayers. Such polymers may be used to generate a variety of differentcolored emissions. Table II provides examples of suitable polymers andthe color of their associated emissions.

                  TABLE II                                                        ______________________________________                                        POLYMER               EMISSION COLOR                                          ______________________________________                                        poly(para-phenylenevinylene)                                                                        blue to green                                           poly(dialkoxyphenylenevinylene)                                                                     red/orange                                              poly(thiophen)        red                                                     poly(phenylene)       blue                                                    poly(phenylacetylene) yellow to red                                           poly(N-vinylcarbazole)                                                                              blue                                                    ______________________________________                                    

The rare earth mixed chelates for use in the present invention includeany lanthanide elements (e.g. La, Pr, Nd, Sm, Eu, and Tb) bonded to abidentate aromatic or heterocyclic ligand. The bidentate ligand servesto transport carriers (e.g. electrons) but does not absorb the emissionenergy. Thus, the bidentate ligands serve to transfer energy to themetal. Examples of the ligand in the rare earth mixed chelates includesalicyladehydes and derivatives thereof, salicyclic acid, quinolates,Schiff base ligands, acetylacetonates, phenanthroline, bipyridine,quinoline and pyridine.

The hole transporting layers 20H, 21H and 22H may be comprised of aporphorinic compound. In addition, the hole transporting layers 20H, 21Hand 22H may have at least one hole transporting aromatic tertiary aminewhich is a compound containing at least one trivalent nitrogen atom thatis bonded only to carbon atoms, at least one of which is a member of anaromatic ring. For example, the aromatic tertiary amine can be anarylamine, such as a monoarylamine, diarylamine, triarylamine, or apolymeric arylamine. Other suitable aromatic tertiary amines, as well asall porphyrinic compounds, are disclosed in Tang et al., U.S. Pat. No.5,294,870, the teachings of which are incorporated herein in theirentirety by reference, provided any of such teachings are notinconsistent with any teaching herein.

The fabrication of a stacked organic LED tricolor pixel according to thepresent invention may be accomplished by either of two processes: ashadow masking process or a dry etching process. Both processes to bedescribed assume, for illustrative purposes, a double heterostructureLED construction, i.e., utilizing only one organic compound layer foreach active emission layer, with light emerging from the bottom glasssubstrate surface. It should be understood that multiple heterojunctionorganic LED's having multiple organic compound layers for each activeemission layer, and/or inverted structures (with light emerging from thetop surface of the stack) can also be fabricated by one skilled in theart making slight modifications to the processes described.

The shadow masking process steps according to the present invention areillustrated in FIGS. 12(A-E). A glass substrate 50 to be coated with alayer of ITO 52 is first cleaned by immersing the substrate 50 for aboutfive minutes in boiling trichloroethylene or a similar chlorinatedhydrocarbon. This is followed by rinsing in acetone for about fiveminutes and then in methyl alcohol for approximately five minutes. Thesubstrate 50 is then blown dry with ultrahigh purity (UHP) nitrogen. Allof the cleaning solvents used are preferably "electronic grade". Afterthe cleaning procedure, the ITO layer 52 is formed on substrate 50 in avacuum using conventional sputtering or electron beam methods.

A blue emitting LED 55 (see FIG. 12B) is then fabricated on the ITOlayer 52 as follows. A shadow mask 73 is placed on predetermined outerportions of the ITO layer 52. The shadow mask 73 and other masks usedduring the shadow masking process should be introduced and removedbetween process steps without exposing the device to moisture, oxygenand other contaminants which would reduce the operational lifetime ofthe device. This may be accomplished by changing masks in an environmentflooded with nitrogen or an inert gas, or by placing the masks remotelyonto the device surface in the vacuum environment by remote handlingtechniques. Through the opening of mask 73, a 50 -100 Å thick holetransporting layer (HTL) 54 and 50 -200 Å thick blue emission layer (EL)56 (shown in FIG. 12B) are sequentially deposited without exposure toair, i.e., in a vacuum. An electron transporting layer (ETL) 58 having athickness preferably of 50-1000 Å is then deposited on EL 56. ETL 58 isthen topped with a semitransparent metal layer 60M which may preferablyconsist of a 10% Ag in 90% Mg layer, or other low work function metal ormetal alloy layer, for example. Layer 60M is very thin, preferably lessthan 100 Å. Layers 54, 56, 58 and 60M may be deposited by any one of anumber of conventional directional deposition techniques such as vaporphase deposition, ion beam deposition, electron beam deposition,sputtering and laser ablation.

An ITO contact layer 60I of about 1000-4000 Å thick is then formed onthe metal layer 60M by means of conventional sputtering or electron beammethods. For convenience herein, the sandwich layers 60M and 60I will bereferred to and shown as a single layer 60, which is substantially thesame as the layer 26 of FIG. 2. The low work function metal portion 60Mof each layer 60 directly contacts the ETL layer beneath it, while theITO layer 60I contacts the HTL layer immediately above it. Note that theentire device fabrication process is best accomplished by maintainingthe vacuum throughout without disturbing the vacuum between steps.

FIG. 12C shows a green emitting LED 65 which is fabricated on top oflayer 60 using substantially the same shadow masking and depositiontechniques as those used to fabricate blue emitting LED 55. LED 65comprises HTL 62, green emission layer 64 and ETL 66. A second thin(<100 Å thick, thin enough to be semi-transparent but not so thin tolose electrical continuity) metal layer 60M is deposited on ETL layer66, followed by another 1000-4000 Å thick ITO layer 60I to form a secondsandwich layer 60.

Shown in FIG. 12D is a red emitting LED 75 fabricated upon layer 60(upon 60I to be specific) using similar shadow masking and metaldeposition methods. Red emitting LED 75 consists of a HTL 70, a redemitting EL 72 and ETL 74. A top sandwich layer 60 of layers 60I and 60Mare then formed on LED 75. As described above for the embodiment of FIG.2, similarly, the top transparent ITO layer 60I can in an alternativeembodiment be replaced by an appropriate metal electrode serving also tofunction as a mirror for reflecting upwardly directed light back throughthe substrate 50, thereby decreasing light losses out of the top of thedevice. Each ETL layer 74, 66 and 58 has a thickness of 50-200 Å; eachHTL layer 54, 62 and 70 is 100-500 Å thick; and each EL layer 56, 64 and72 is 50-1000 Å thick. For optimum brightness and efficiency, each ofthe layers including the ITO/metal layers should be kept as close aspossible towards the lower end of the above ranges.

The formation of electrical contacts 51 and 59 on ITO layer 52, andelectrical contacts 88, 89, 92, 94 and 96 on the ITO portion 60I ofITO/metal layers 60 is then preferably accomplished in one step. Theseelectrical contacts may be indium, platinum, gold, silver orcombinations such as Ti/Pt/Au, Cr/Au or Mg/Ag. They may be deposited byvapor deposition or other suitable metal deposition techniques aftermasking off the rest of the device.

The final step in the shadow masking process is to overcoat the entiredevice with an insulating layer 97 as shown in FIG. 12E, with theexception of all the metal contacts 51, 59, 88, 89, 92, 94 and 96 whichare masked. Insulating layer 97 is impervious to moisture, oxygen andother contaminants thereby preventing contamination of the LED's.Insulating layer 97 may be SiO₂, a silicon nitride such as Si₂ N₃ orother insulator deposited by electron-beam, sputtering, or pyroliticallyenhanced or plasma enhanced CVD. The deposition technique used shouldnot elevate the device temperature above 120° C. inasmuch as these hightemperatures may degrade the LED characteristics.

The dry etching process for fabricating the LED stack according to theinvention is illustrated in FIGS. 13(A-F). Referring to FIG. 13A, aglass substrate 102 is first cleaned in the same manner as in theshadow-mask process described above. An ITO layer 101 is then depositedon the glass substrate 102 in a vacuum using conventional sputtering orelectron beam methods. An HTL 104, blue EL 105, ETL 106 and sandwichlayer comprising metal layer 107M and ITO layer 107I, all of generallythe same thicknesses as in the shadow-masking process, are thendeposited over the full surface of the ITO layer 101, using eitherconventional vacuum deposition, or in the case of polymers spin or spraycoating techniques. ITO/metal sandwich layer 107 consists of a less than100 Å thick, low work function metal layer 107M deposited directly onthe ETL layer 106, and a 1000-4000 Å thick ITO layer 107I on the metallayer 107M. On the entire top surface of ITO layer 107I, a 1000 Å-2000 Åthick layer of silicon nitride or silicon dioxide masking material 108is deposited using low temperature plasma CVD. A positive photoresistlayer 109 such as HPR 1400 J is then spun-on the silicon nitride layer108. As shown in FIG. 13B the outer portions 110 (see FIG. 13A) of thephotoresist layer 109 are exposed and removed using standardphotolithographic processes. The exposed outer portions 110 correspondto the areas where the bottom ITO layer 101 is to be exposed andelectrically contacted. Referring to FIG. 13C, the outer regions 111(defined in FIG. 13B) of the silicon nitride layer 108 corresponding tothe removed photoresist areas, are removed using a CF₄ :O₂ plasma. Then,using an ion milling technique or another plasma etch, the exposed outerportions of ITO/metal layers 107I and 107M are removed. An O₂ plasma isthen employed to sequentially remove the corresponding exposed outerportion of the ETL layer 106, EL layer 105, and HTL layer 104,respectively, and also to remove the remaining photoresist layer 109shown in FIG. 13D. Finally, a CF₄ :O₂ plasma is again applied to removethe silicon nitride mask 108, with the resulting blue LED configurationshown in FIG. 13D.

The same sequence of dry etching process steps is used to fabricate agreen LED 115 atop the blue LED, except that SiNx 150 is overlaid asshown, followed by a photoresist mask 113 as shown in FIG. 13E to maskthe outer portion of ITO layer 101. Then the deposition of HTL layer114, green EL layer 116, and so on is performed (see FIG. 13F). The samephotolithography and etching techniques used for blue LED fabricationare then employed to complete the formation of the green LED 115. Thered LED 117 is then formed atop the green LED using substantially thesame dry etching process. A passivation layer 119 similar to layer 97 ofFIG. 12E is then deposited over the LED stack with suitable patterningto expose electrical contacts, as was described for the shadow maskingprocess. A photoresist mask is used to allow dry etching of holes inpassiuation layer 119. Next, metal 152 is deposited in the holes. Afinal photoresist layer and excess metal is removed by a "lift-off"process.

Following the LED stack fabrication, whether performed by a shadow mask,dry-etching or other method, the stack must be properly packaged toachieve acceptable device performance and reliability. FIGS. 14(A-C)illustrate embodiments of the invention for faciliting packaging, andfor providing a hermetic package for up to four of the multicolor LEDdevices of the invention, for example. The same reference numerals usedin FIGS. 14 (A-B) indicate the identical respective features as in FIG.12E. The package may also be used with the nearly identical structure ofFIG. 13F. Referring to FIG. 14A, after overcoating the entire devicewith an insulating layer 97, such as SiNx for example, access holes 120,122, and 124 are formed using known etching/photomasking techniques toexpose the topmost metal layers 60M', 60M", and 60M"', for the blue,green, and red LED (organic light emitting diode) devices, respectively,in this example. Thereafter, suitable metal circuit paths 126, 128, and130 (typically of gold material), are deposited in a path from theexposed metal layers 60M', 60M", and 60M"', respectively, to edgelocated indium solder bumps 132, 133, and 134, respectively, usingconventional processing steps. Similarly, an anode electrode terminationis provided via the metal (Au, for example) circuit path 135 formed tohave an inner end contacting ITO layer 52, and an outer end terminatingat an edge located indium solder bump 136, all provided via conventionalprocessing. The device is then overcoated with additional insulatingmaterial such as SiNx to form an insulated covering with solder bumps132, 133,134, and 136 being exposed along one edge. In this manner, theorganic LED device can be readily packaged using conventionaltechniques, or the packaging embodiment of the invention as describedimmediately below.

A method for making four multicolor LED devices on a common substrate 50in a packaged configuration will now be described, with reference toFIGS. 14A, 14B, and 14C, respectively, for another embodiment of theinvention. The starting material includes a glass substrate 50 coatedwith an overlayer of indium tin oxide (ITO) 52. The following steps areused to obtain the packaged multicolor organic LED array:

1. Mask ITO layer 52 to deposit an SiO₂ layer 138 in a concentric squareband ring pattern, in this example (some other pattern can be employed),on top of ITO layer 52 using conventional techniques.

2. Form four three-color LED stacks sharing common layers in region 140on the ITO layer 52 using methods as taught above for obtaining, forexample, either of the structures of FIGS. 12E or 13F, and 14A.

3. Deposit via shadow masking metal contacts 170 through 181; eachterminating at exterior ends on SiO₂ layer 138, for providing externalelectrical connecting or bonding pads 170 through 181', respectively.Note that contacts 126, 128, and 130 in FIG. 14A are the same as everysuccessive three of contacts 170-181, respectively. Each group of threecontacts, namely 170 through 172, 173 through 175, 176 through 178, and179 through 181, terminate at their interior or other ends to provide anelectrical connection with the metal layers 60M', 60M", 60M"',respectively, of each of the four organic LED devices, respectively.Another metal contact 182 is deposited via shadow masking on an edge ofITO layer 52 common to all four of the LED devices, for providing acommon anode connection, in this example. Note that if throughappropriate masking and etching the four LED devices are made incompletely independent layers, four anode contacts, respectively, willhave to be provided for the latter array so that it can be operated in amultiplexed manner. The multicolor LED array being described in thisexample is a non-multiplexed array.

4. Deposit via shadow masking, for example, a second SiO₂ layer 184 in acontinuous band or ring leaving exposed bonding pads 170' through 181',using either sputtering, or plasma enhanced CVD, or electron beamdeposition, for example.

5. Deposit Pb--Sn or other low temperature melting solder in acontinuous band or ring 186 on top of the second SiO₂ layer or band 184.

6. Deposit on the bottom of a cover glass 188 a metal ring 190 to becoincident with the solder seal ring 186.

7. Place the assembly in an inert gas atmosphere, such as dry nitrogen,and apply heat to melt solder ring 186 to obtain an air tight seal, withthe inert gas trapped in interior region 192.

8. Install cover glass 188 over the assembly, as shown in FIG. 14B, withmetal ring 190 abutting against the solder ring 186.

Referring to FIG. 15, there is shown a display 194 which is an RGBorganic LED display. The dots 195 are ellipsis. A complete display as194 comprises a plurality of pixels such as 196. The pixels are arrangedas a XY matrix to cover the entire surface area of a glass sheet coatedwith ITO. Each pixel includes a stacked LED structure as that shown inFIG. 2. Instead of having fixed bias means as batteries 30, 31 and 32(FIG. 2) each of the lines of terminals designated in FIG. 2 as blue(B), green (G) and red (R) are brought out and coupled to suitablehorizontal and vertical scan processors 197 and 198, respectively, allunder control of a display generator 199 which may be a TV unit.Accordingly, each matrix of LED's has at least two axes (x,y), and eachLED is at the intersection of at least two of the axes. Also, the x-axismay represent a horizontal axis, and the y-axis a vertical axis. It iswell known how to convert television signals such as the NTSC signalsinto the color components R, G and B for color displays. Monitors forcomputers which utilize red, green and blue for primary colors are alsowell known. The drive and control of such devices by vertical andhorizontal scanning techniques are also known. The entire array of pixelstructures deposited over the surface of the display is scannedemploying typical XY scanning techniques as using XY addressing. Thesetechniques are used in active matrix displays.

One can use pulse width modulation to selectively energize the red,green and blue inputs of each of the DH LED pixels according to desiredsignal content. In this manner, each of the LED's in each line of thedisplay are selectively accessed and addressed and are biased by manymeans such as by pulse width modulation signals or by staircasegenerated voltages to enable these devices to emit single colors ormultiple colors, so that light emitted from said structures creates animage having a predetermined shape and color. Also, one can seriallyscan each of the xy axes, and serially energize selected ones of theLED's in the matrix to emit light for producing an image with colorscreated serially vertically. Selected ones of the LED's may besimultaneously energized.

As indicated above, the vertical layering technique shown in FIG. 2allows the fabrication of the three color DH LED pixel within extremelysmall areas. This allows one to provide high definition displays such asdisplays that have 300 to 600 lines per inch resolution or greater. Suchhigh resolution would not be obtainable using prior art techniques inwhich the organic emission layers or fluorescent mediums generating thedifferent colors are laterally spaced from one another.

Based on modern standards one can provide a LED device as shown in FIG.2 with an effective area small enough to enable hundreds of pixel diodesto be stacked vertically and horizontally within the area of a squareinch. Therefore, the fabrication techniques enables one to achieveextremely high resolution with high light intensity.

In FIG. 16, another embodiment of the invention is shown for amulticolor LED device including the stacking of up to N individual LEDs,where N is an integer number 1,2,3 . . . N. Depending upon the state ofthe technology at any future time, N will have a practical limit. Thestacked N levels of LED's can, for example, be provided using either theshadow masking process steps previously described for FIGS. 12 (A-E), orthe dry etching process illustrated in FIGS. 13A through 13F. The baseor bottom portion of the stacked array of FIG. 16 is a glass substrate102 as shown in FIG. 13F, for example, with an ITO layer 101 formed oversubstrate 102. The immediately overlying first LED device, and followingLED devices in this example, each include in succession over ITO layer101 an HTL layer 154, an EL layer 156, an ETL layer 158, a metal layer160, and an ITO layer 162. The N^(th) level LED device 164 furtherincludes a topmost metal layer (see layer 152 of FIG. 13F) formed overthe uppermost ITO layer 162 thereof. A passivation layer 119 isdeposited over the stack, as in the color stack of FIG. 13F. Thematerial for each EL layer 156 of each LED device is selected forproviding a particular color for the associated LED. As in the threecolor device, shorter wavelength (blue) devices must lie lower in thestack than the longer wavelength (red) devices to avoid opticalabsorption by the red emitting layers. The color selected for eachrespective LED and the actual number of stacked LEDs are dependent uponthe particular application, and the desired colors and shadingcapability to be provided. Such multi-color devices can also be used inoptical communications networks, where each different optical channel istransmitted using a different wavelength emitted from a given device inthe stack. The inherently concentric nature of the emitted light allowsfor coupling of several wavelengths into a single optical transmissionfiber. In practical such stacked arrays, access holes are formed down tothe ITO layer 162 of each device followed by the deposition ofappropriate metallization for facilitating packaging and electricalconnection to each of the LED devices in the stack, in a manner similarto that described for the stacked multicolor LED device of FIGS. 14A,14B, and 14C, for example.

This device can be used to provide a low cost, high resolution, highbrightness full color, flat panel display of any size. This widens thescope of this invention to displays as small as a few millimeters to thesize of a building but to a practice limit. The images created on thedisplay could be text or illustrations in full color, in any resolutiondepending on the size of the individual LED's.

Those with skill in the art may recognize various modifications to theembodiments of the invention described and illustrated herein. Suchmodifications are meant to be covered by the spirit and scope of theappended claims. For example, a multicolor stacked LED device, such asthe above-described three color device of FIG. 2, in another embodimentof the invention can be provided by forming LED 20 from a polymer deviceas shown in FIG. 1C, or from a deposited metal phosphonate film, ratherthan having all three layers laid down in vacuo. The two remainingstacked LED's would be formed by vapor deposition.

What is claimed is:
 1. A multicolor light emitting device (LED)structure, comprising:a plurality of at least first and second lightemitting organic devices (LEDs) stacked one upon the other, to form alayered structure, with each LED separated one from the other by atransparent electrically conductive layer to enable each device toreceive a separate bias potential to operate to emit light through thestack.
 2. The multicolor light emitting device structure of claim 1,wherein each of said LED's emits a different wavelength of light andtherefore a different color when biased.
 3. The multicolor lightemitting device of claim 2, wherein the longest wavelength LED is on topof the stack in the vertical direction followed by successively shorterwavelength LED's, with shortest wavelength LED on the bottom of thestack.
 4. The multicolor light emitting device structure of claim 1,including at least first through third light emitting devices stackedupon one another, respectively.
 5. The multicolor light emitting devicestructure of claim 4, wherein said first device emits the color blue(B), said second device emits the color green (G) and third device emitsthe color red (R).
 6. The multicolor light emitting device structure ofclaim 5, wherein said devices are stacked in the following sequencealong the vertical axis starting from a bottom point and directedupward, wherein the first device emits a blue color, and has the seconddevice for emitting a green color located on top of the upper surface ofsaid blue emitting device, with the third device for emitting a redcolor located on top of the upper surface of said green emitting device,whereby said blue emitting device of the shortest wavelength is at thebottom with the red emitting device of the longest wavelength on topwhen the structure is aligned vertically.
 7. The multicolor lightemitting device structure of claim 4, wherein said at least first,second, and third organic LED's are stacked in successive order over acommon substrate.
 8. The multicolor light emitting device structure ofclaim 7, wherein said substrate is at the bottom of said LED structure,and a topmost layer of said third organic LED consists of indium tinoxide (ITO) material serving as a contact for an underlying metalmaterial layer.
 9. The multicolor light emitting device structure ofclaim 1, wherein each LED device is a transparent double heterostructure(DH) device fabricated from organic materials.
 10. The multicolor lightemitting device structure of claim 9, wherein said transparentelectrically conductive layer is an indium tin oxide (ITO).
 11. Themulticolor light emitting device structure of claim 9, wherein saidtransparent electrically conductive layer comprises a metallic layerhaving a work function less than four electron volts, and an ITO layeron said metallic layer.
 12. The multicolor light emitting devicestructure of claim 9, wherein said organic material is selected from thegroup consisting of trivalent metal quinolate complexes, trivalent metalbridged quinolate complexes, Schiff base divalent metal complexes, tin(iv) metal complexes, metal acetylacetonate complexes, metal bidentateligand complexes, bisphosphonates, divalent metal maleonitriledithiolatecomplexes, molecular charge transfer complexes, aromatic andheterocyclic polymers, and rare earth mixed chelates.
 13. The multicolorlight emitting device structure of claim 12 wherein the trivalent metalquinolate complexes have the following formula ##STR1## wherein R isselected from the group consisting of hydrogen, alkyl, aryl and aheterocyclic group, L represents a ligand selected from the groupconsisting of picolylmethylketone; salicylaldehyde; a group of theformula R(O)CO-- wherein R is as defined above; halogen; a group of theformula RO-- wherein R is as defined above; and quinolates.
 14. Themulticolor light emitting device structure of claim 12 wherein the metalbidentate ligand complexes have the following formula:

    MDL.sup.4.sub.2

wherein M is selected from trivalent metals of Groups 3-13 of thePeriodic Table and the Lanthanides, D is a bidentate ligand and L⁴ isselected from the group consisting of acetylacetonate; compounds of theformula OR³ R wherein R³ is Si or C and R is selected from the groupconsisting of hydrogen, alkyl, aryl, a heterocyclic group;3,5-di(t-butyl) phenol; 2,6-di(t-butyl) phenol; 2,6-di(t-butyl) cresoland a compound of the formula ##STR2##
 15. The multicolor light emittingdevice structure of claim 14 wherein D is selected from the groupconsisting of 2-picolylketones, 2-quinaldylketones and 2-(o-phenoxy)pyridineketones.
 16. The multicolor light emitting device structure ofclaim 12 wherein the Schiff base divalent metal complexes are selectedfrom those having the formula ##STR3## wherein M¹ is a divalent metalchosen from Groups 2-12 of the Periodic Table, R¹ is selected from thegroup consisting of ##STR4## ##STR5## wherein X is selected from thegroup consisting of hydrogen, alkyl, alkoxy each having 1 to 8 carbonatoms, aryl, a heterocyclic group, phosphino, halogen and amine.
 17. Themulticolor light emitting device structure of claim 12 wherein thearomatic and heterocyclic polymers are selected from the groupconsisting of poly (para-phenylene vinylene), poly (dialkoxyphenylenevinylene), poly (thiophene), poly (phenylene), poly (phenylacetylene)and poly (N-vinylcarbazole).
 18. The multicolor light emitting devicestructure of claim 12 wherein the rare earth mixed chelates comprise aLanthanide bonded to a bidentate aromatic or heterocyclic group.
 19. Themulticolor light emitting device structure of claim 18 wherein thebidentate aromatic or heterocyclic group is selected from the groupconsisting of salicylaldehydes, salicyclic acid, quinolates, Schiff baseligands, acetylacetonates, phenanthroline, bipyridine, quinoline andpyridine.
 20. The multicolor light emitting device structure of claim 18wherein the divalent metal maleonitriledithiolate complexes have theformula ##STR6## wherein M³ is a metal having a +2 charge, Y¹ isselected from the group consisting of cyano, and phenyl, and L⁵ is agroup having no charge.
 21. The multicolor light emitting devicestructure of claim 20 wherein L⁵ is a group of the formula P(OR)₃ orP(R)₃ wherein R is selected from the group consisting of hydrogen,alkyl, aryl and a heterocyclic group.
 22. The multicolor light emittingdevice structure of claim 12 wherein the bisphosphonates have theformula M² _(x) (O₃ P-organic-PO₃)_(y) wherein M² is a metal ion andorganic represents an aromatic or heterocyclic fluorescent compoundbifunctionalized with phosphonate groups.
 23. The multicolor lightemitting device structure of claim 12 wherein the trivalent metalbridged quinolate complexes have the formula ##STR7## wherein M is atrivalent metal ion and Z is selected from SiR or P=O wherein R isselected from the group consisting of hydrogen, alkyl, aryl, or aheterocyclic group.
 24. The multicolor light emitting device structureof claim 12 wherein the tin (iv) metal complexes have the formula SnL¹ ₂L² ₂ wherein L¹ is selected from the group consisting ofsalicylaldehydes, salicyclic acid, and quinolates and L² is selectedfrom the group consisting of alkyl, aryl and a heterocyclic group. 25.The multicolor light emitting device structure of claim 12 wherein themolecular charge transfer complexes comprise an electron acceptorcomplexed with an electron donor.
 26. The multicolor light emittingdevice structure of claim 1, wherein each LED device is a transparentsingle heterostructure device fabricated from organic materials.
 27. Themulticolor light emitting device structure of claim 2, wherein saidtransparent electrically conductive layer is an indium tin oxide (ITO).28. The multicolor light emitting device structure of claim 1, whereinsaid devices are stacked in an order dependent upon and in accordancewith their respective emission wavelength and absorptioncharacteristics.
 29. The light emitting device structure of claim 1,wherein each of said LEDs emit substantially the same wavelength oflight.
 30. The multicolor LED structure of claim 1, wherein saidplurality of at least first and second LEDs are stacked over a commonsubstrate.
 31. The multicolor LED structure of claim 1, furtherincluding a reflective metal layer over one end of the stack of saidplurality of a LEDs.
 32. The multicolor LED structure of claim 31,further including a layer of anti-reflecting material disposed over theother end of said plurality of at least first and second LEDs.
 33. Themulticolor LED structure of claim 1, further including a layer ofanti-reflecting material disposed over one end of the stack of saidplurality of at least first and second LEDs.
 34. A multicolor lightemitting device structure comprisinga transparent substrate layer havingdeposited on a surface a first transparent electrically conductivecoating. a first light emitting device deposited on said firsttransparent electrically conductive coating; a second transparentelectrically conductive coating deposited on the surface of said firstdevice not in contact with said first coating; a second light emittingdevice deposited on the surface of said second coating; a thirdtransparent electrically conductive coating deposited on the surface ofsaid second device not in contact with said second coating; a thirdlight emitting device deposited on the surface of said third coating;said first, second, and light emitting devices each including an organicemission layer for emitting light of a desired color, respectively; anda fourth electrically conductive coating deposited on the surface ofsaid third device not in contact with said third coating.
 35. Themulticolor light emitting device structure of claim 34, wherein saidfirst, second, third and fourth electrically conductive coatings areadapted to receive individual sources of biasing potential,respectively.
 36. The multicolor light emitting device structure ofclaim 34, wherein said devices and electrically conductive layers aredeposited to form a staircase profile, with said transparent substratebeing of a greater length than said first device, with said first devicebeing of a greater length than said second device, with said seconddevice of a greater length than said third device, wherein each step iscovered by said respective electrically conductive coating adapted forapplying operating potentials to said device structures, and whereinsaid first through third transparent electrically conductive coatingsallow light emitted by any of said devices, respectively, to passthrough said transparent substrate layer.
 37. The multicolor lightemitting device structure of claim 34, wherein said forth electricallyconductive coating, includes a a metal layer that reflects upwarddirected light back to said substrate.
 38. The multicolor light emittingdevice structure of claim 37, wherein said fourth electricallyconductive coating further includes an indium tin oxide (ITO) layerbetween said metal layer and said surface of said third device not incontact with said third coating, said ITO layer serving as a contact foran underlying metal material layer of said third light emitting diodedevice.
 39. The multicolor light emitting device structure of claim 34,wherein said transparent substrate is glass, said first conductivecoating is indium tin oxide (ITO), and each of said second, third andfourth conductive coatings are each comprised of an ITO layer disposedon a low work function metal layer, respectively.
 40. The multicolorlight emitting device structure of claim 34, wherein each of saiddevices are double heterostructures (DH), with said first deviceoperative when biased to emit blue light (B), said second deviceoperative when biased to emit green light (G), said third deviceoperative when biased to emit red light (R).
 41. The multicolor lightemitting device structure of claim 40, wherein each DH structure iscomprised of organic compounds.
 42. The multicolor light emitting devicestructure of claim 34, wherein each of said devices are singleheterostructures (SH), with said first device operative when biased toemit blue light (B), said second device operative when biased to emitgreen light (G), and said third device operative when biased to emit redlight (R).
 43. The multicolor LED structure of claim 42, whereineach ofsaid single heterostructures (SH) is comprised of organic material. 44.The multicolor light emitting device structure of claim 34, wherein eachof said devices are polymer structures, with said first device operativewhen biased to emit blue light (B), said second device operative whenbiased to emit green light (G), and said third device operative whenbiased to emit red light (R).
 45. The light emitting device structure ofclaim 34, wherein each of said devices are operative when biased to emitlight of substantially the same wavelength.
 46. The multicolor lightemitting device structure of claim 34, further including a layer ofanti-reflecting material deposited between said transparent substratelayer and said first transparent electrically conductive coating. 47.The multicolor light emitting device structure of claim 46, furtherincluding a reflective metal layer between said fourth electricallyconductive coating and the surface of said third device not in contactwith said third coating for reflecting upward directed light back tosaid transparent substrate layer.
 48. A method of fabricating amulticolor, light emitting device (LED) structure comprising the stepsof:forming a first transparent electrically conductive layer upon atransparent substrate; depositing a first hole transporting layer uponsaid first transparent electrically conductive layer, depositing a firstorganic emission layer upon said first hole transporting layer toprovide a first emission color; depositing first electron transportinglayer upon said first emission layer; depositing a second transparentelectrically conductive layer upon said first electron transportinglayer, said second transparent electrically conductive layer adapted toreceive a first bias potential; depositing a second hole transportinglayer upon said second transparent electrically conductive layer;depositing a second organic emission layer upon said second holetransporting layer to provide a second emission color; depositing asecond electron transporting layer upon said second emission layer; anddepositing a third transparent electrically conductive layer upon saidsecond electron transporting layer, said third transparent electricallyconductive layer adapted to receive a second bias potential.
 49. Themethod of claim 48, further including the step of shadow masking aregion of said first transparent electrically conductive prior todepositing said first hole transporting layer expose said region of saidfirst transparent, electrically conductive layer thereby enabling saidfirst bias potential to be applied between said second transparentelectrically conductive layer and said region of said first transparentelectrically conductive layer.
 50. The method of claim 48, furtherincluding the step of etching away a region of said first holetransporting layer to expose a portion of said first transparentelectrically conductive layer thereby enabling said first bias potentialto be applied between said second transparent electrically conductivelayer and said exposed portion of said first transparent electricallyconductive layer.
 51. The method of claim 48, further including the stepof forming a layer of anti-reflecting material between said firsttransparent electrically conductive layer and said transparentsubstrate.
 52. The method of claim 51, further including the step offorming a metal layer on said third transparent electrically conductivelayer, for reflecting upward directed right back to said substrate. 53.The method claim 48, further including the step of forming a metal layeron said third transparent electrically conductive layer, for reflectingupward directed light back to said substrate.
 54. A multicolor,energizable, light emitting structure, comprising:at least three layersof electrically conductive material; a transparent, energizable, lightemitting device including an emission layer of organic material, (OLED),being disposed between adjacent ones of said layers of electricallyconducive material, respectively, so that said OLEDs are stacked on eachother with one of said layers of electrically conductive materialdisposed between each two of said OLEDs and the other layers ofelectrically conductive material are disposed on the outside of saidOLEDs; said layers of electrically conductive material disposed betweenadjacent ones of said OLEDs and one of said outside layers beingsubstantially transparent; and means on each of said layers ofelectrically conductive material for being connected to a bias forselectively energizing each of said OLEDs.
 55. The structure of claim54, wherein each of said OLEDs emits a different color.
 56. Thestructure of claim 55, wherein said OLEDs are stacked in a verticalarray.
 57. The structure of claim 56, further including:a third OLED insaid stack; the middle OLED being operative to emit light of apredetermined wavelength; the top OLED being operative to emit light ofa longer wavelength than the predetermined wavelength; and the bottomOLED being operative to emit light of a shorter wavelength than thepredetermined wavelength.
 58. The structure of claim 56, furtherincluding:a transparent substrate; said stack of OLEDs and layers ofelectronically conductive material being supported by transparentsubstrate in an order that corresponds to the length of the light wavethat said OLEDs emit; and said OLED emitting the shortest wavelength isclosest to said transparent substrate so that the light emitted fromeach of said OLEDs when it is energized is transmitted through the otherOLEDs and through said through said transparent substrate.
 59. Thestructure of claim 58, further including:a layer of anti-reflectingmaterial disposed between said OLED emitting the shortest wavelength andsaid transparent substrate so that the light emitted from each of saidOLEDs when it is energized is not reflected from said transparentsubstrate.
 60. The structure of claim 58, further including:a layer ofreflective material adjacent said OLED emitting the longest wavelengthfor reflecting light emitted from said OLED back through said substrate.61. The structure of claim 54, wherein said layer of electricallyconductive material includes indium-tin-oxide (ITO) and a metal.
 62. Thestructure of claim 61, wherein said metal has a work function of lessthan four electron volts.
 63. The structure of claim 54, furtherincluding:a transparent substrate; said stack of OLEDs and layers ofelectrically conductive material being supported by said transparentsubstrate in an order that corresponds to the length of the light wavethat said OLEDs emit; and said emitting the shortest wavelength iscloset to said transparent substrate for maximizing light emitted fromsaid structure from each of said OLEDs respectively, when energized. 64.The structure of claim 63, further including:a layer of anti-reflectingmaterial disposed between said OLED emitting the shortest wavelength andsaid transparent substrate so that the light emitted from each of saidOLEDs when it is energized is not reflected from said transparentsubstrate.
 65. The structure of claim 64, further including:a layer ofreflective material adjacent said OLED emitting the longest wavelength.66. The structure of claim 64, wherein said layer of electricallyconductive material includes an indium-tin oxide (ITO) layer and a metallayer.
 67. The structure of claim 66, wherein said metal has a workfunction of less than four electron volts.
 68. The structure of claim63, further including:a layer of reflective material adjacent said OLEDemitting the longest wavelength for reflecting light emitted from saidLED back through said substrate.
 69. The structure of claim 54, whereineach of said OLEDs is a double heterostructure.
 70. The structure ofclaim 69 wherein each of said LEDs is comprised of organic material. 71.The structure of claim 54, wherein each of said OLEDs is a singleheterostructure.
 72. The structure of claim 71, wherein each of saidLEDs is comprised of organic material.
 73. The structure of claim 54,wherein each of said OLEDs emit light of substantially the same color.74. An energizable, light emitting structure, comprising:a transparentsubstrate; a first layer of transparent, electrically conductivematerial supported on said substrate; a transparent, energizable, lightemitting device (LED) supported on said first layer of transparent,electrically conductive material, said LED including an emission layerof organic material; a second layer of transparent electricallyconductive material supported by said LED; and said LED being operativeto produce light and transmit it through said transparent substrate whenenergized.
 75. The structure of claim 74, wherein said first and secondlayers comprise indium-tin-oxide.
 76. The structure of claim 75, whereinsaid second layer further comprises a layer of metal that has a workfunction that is less than four electron volts.
 77. The structure ofclaim 76, wherein said metal is from the group consisting of magnesium,arsenic and magnesium-gold alloy.
 78. The structure of claim 74, furtherincluding:said second layer of electrically conductive material beingsubstantially transparent; a second transparent, energizable, lightemitting device (LED) supported on said second LED including an emissionlayer of organic material; a third layer of electrically conductivematerial supported by said second LED; and said second LED beingoperative to produce light and transmit it through said first LED andthrough said transparent substrate when energized.
 79. The energizablelight emitting structure of claim 74, wherein said emission layerincludes at least one material is selected from the group consisting oftrivalent metal quinolate complexes, trivalent metal bridged quinolatecomplexes, Schiff base divalent metal complexes, tin (iv) metalcomplexes, metal acetylacetonate complexes, metal bidentate ligandcomplexes, bisphosphonates, divalent metal maleonitriledithiolatecomplexes, molecular charge transfer complexes, aromatic andheterocyclic polymers and rare earth mixed chelates.
 80. The structureof claim 74, wherein said LED is double heterostructure (DH).
 81. Thestructure of claim 80 wherein said LED is comprised of organic material.82. The structure of claim 74 wherein said LED is single heterostructure(SH).
 83. The structure of claim 82, wherein said LED is comprised oforganic material.
 84. The structure of claim 74, further including alayer of anti-reflecting material between said transparent substrate andsaid first layer of transparent electrically conductive material. 85.The structure of claim 84, further including a metal layer on saidsecond layer of transparent electrically conductive material, forreflecting light emitted from said LED back though said transparentsubstrate.
 86. The structure of claim 74, further including a reflectivemetal layer on said second layer of transparent electrically conductivematerial, for reflecting light emitted from said LED back through saidtransparent substrate.
 87. A method of fabricating a multicolor,energizable light emitting structure, comprising the steps of:providinga transparent substrate; providing a first substantially transparentelectrically conductive layer on said transparent substrate; providing afirst transparent, light emitting diode (LED) on said substrate, saidfirst LED including an emission layer of organic material and, beingoperable when energized to emit light of a first predeterminedwavelength; providing a second substantially transparent, electricallyconductive layer on said first LED; providing a second transparent,light emitting diode (LED) on said second substantially transparent,electrically conductive layer, said second LED including an emissionlayer of organic material and being operable when energized to emitlight of a second predetermined wavelength, that is longer than saidfirst predetermined wavelength; and an electrically conductive layer onsaid second (LED).
 88. A method as in claim 87, whereinsaid steps ofproviding said first and second LEDs comprises the steps of forming eachof said LEDs by: depositing a hole transporting layer on said first andsecond substantially transparent, electrically conductive layers;depositing an emission layer on each of said hole transporting layers;and depositing an electron transporting layer on each of said emissionlayers.
 89. The method of claim 88, wherein each of said emission layersincludes a material selected from the group consisting of trivalentmetal quinolate complexes, trivalent metal bridged quinolate complexes,Schiff base divalent metal complexes, tin (iv) metal complexes, metalacetylacetonate complexes, metal bidentate ligand complexes,bisphosphonates, divalent metal maleonitriledithiolate complexes,molecular charge transfer complexes, aromatic and heterocyclic polymersand rare earth mixed chelates.
 90. A method as in claim 89, wherein eachof said substantially transparent electrically conductive layers andsaid layer of electrically conductive layer are comprised ofindium-tin-oxide.
 91. A method as in claim 88, further including:thestep of providing a layer of substantially transparent metal betweensaid LEDs; and a layer of said substantially transparent electricallyconductive material on each of said layers of substantially transparentmetal.
 92. A method as in claim 91, wherein said metal has a workfunction of less than about four electron volts.
 93. A method as inclaim 91, wherein said metal is from the group consisting of magnesium,arsenic and magnesium/gold alloy.
 94. A method as in claim 87, whereinsaid layer of electrically conductive material has a reflective surfacefor reflecting light emitted from said LEDs through said transparentsubstrate.
 95. A method of claim 94, further including the step ofproviding a layer of anti-reflective material between said substrate andsaid first transparent electrically conductive layer.
 96. A method as inclaim 87, further including the step of:providing an electrical contacton each of said layers of substantially transparent, electricallyconductive material, and on said layer of electrically conductivematerial so that each of said layers can be connected to a source ofbias potential.
 97. A method as in claim 87, further including the stepof providing a layer of anti-reflective material between said substrateand said first transparent electrically conductive layer.